1. Field of the Invention
The present invention relates to continuous-wave frequency-modulated (FM-CW) ranging systems and, in particular, to a modulation rate and frequency excursion drift compensator for use therein.
2. Description of the Prior Art
Frequency-modulated continuous-wave (FM-CW) radar ranging systems are well known in the art. In such systems, an interrogation signal, frequency modulated with a given modulation waveform, is transmitted towards a target and reflected therefrom back to the interrogating unit. The reflected signal as received at the interrogating unit is delayed in time, and hence shifted in frequency, from the instantaneous interrogation signal by an amount (.tau.) proportional to the range (R) of the target. For example, when the interrogation signal is modulated in accordance with a triangular waveform having a peak value of .DELTA.F and a period of 1/f.sub.m, the frequency shift or difference frequency f.sub.R, as generated by a suitable filtered mixer receptive of the reflected signal and a signal indicative of the interrogation signal, is equal to the time derivative of the frequency of the interrogation signal times the round trip time delay .tau. and may be expressed as: ##EQU1## where C is the speed of light. The range between the target and the interrogating unit is determined by measurement of the frequency shift f.sub.R.
In practice, such FM-CW radar ranging systems have been plagued with errors due to drifts in the maximum frequency excursion (.DELTA.F) and period (1/f.sub.m) of the frequency modulation waveform of the interrogation signal and by drifts in the system clock. Such drifts are often caused merely by changes in ambient conditions, such as temperature changes, power supply variations and timing circuit drifts. The aforementioned copending applications Ser. Nos. 536,558 and 536,559, respectively, by Kaplan and Ritzie, disclose signal processors for FM-CW ranging systems which, in addition to substantially reducing quantization error, make provisions for compensating for drifts in the period (1/f.sub.m) of the modulation waveform and in the system clock. However, no provision is made for compensating for drifts in the frequency excursion (.DELTA.F) of the modulation waveform.
The prior art has typically used one of two methods of compensating for drifts in the frequency excursion. In one such prior art method, a delay line of known length is connected between the transmitter and receiver antenna feed lines. The time delayed signal, generated by the delay line from a portion of the interrogation signal simulates a reflected signal from a target at a predetermined range. The processor is then adjusted to display the predetermined range. Such a method is disadvantageous in that any subsequent changes in the modulation frequency or amplitude require recalibration of the system. A second prior art method utilizes a delay line in conjunction with frequency discriminators in a complex closed loop stabilizing system. Such a system is described in "The Radar Handbook", M. I. Skolnik, McGraw-Hill, 1970, Chapter 16, page 32. However, frequency discriminators are relatively expensive and not suitable for large scale usage, such as in automotive collision avoidance radars. Such frequency discriminator systems maintain constant only the frequency excursion .DELTA.F, and, moreover, are typically not accurate if the frequency f.sub.m of the modulation waveform drifts, thus requiring that the ranging system employ, for example, relatively expensive crystal oscillators, temperature controlled ovens, or both.